Online　Question　and　answer　(Q&A)　communities　are　the　common　and　famous　platforms　to　learn　and　share　knowledge　and　are　very　useful　for　every　knowledge　seeker.　Less　knowledge　contribution　is　a　critical　issue　for　the　sustainability　and　future　of　these　platforms.　The　motivation　of　inactive　users　to　participate　in　Q&A　communities　is　a　real　challenge.　Based　on　the　social　cognitive　and　social　exchange　theory,　we　have　studied　the　knowledge　contribution　patterns　of　active　and　consistent　StackOverflow　users　over　the　last　eleven　years.　We　have　used　a　difference　generalized　method　of　moments　estimator　to　estimate　the　proposed　model.　Results　revealed　that　reciprocation　of　knowledge　and　social　interaction　positively,　whereas　knowledge　seeking　of　active　and　consistent　users　negatively　influences　knowledge　contribution.　Peer　recognition　and　repudiation　have　partially　positive　and　negative　effects　on　users＇　knowledge　contribution.　This　research　offers　theoretical　and　practical　suggestions　to　encourage　people　to　contribute　their　knowledge　to　online　Q&A　communities.